Process to produce polymers

ABSTRACT

A process to produce ethylene polymers is provided. Particularly, a process to produce ethylene polymers having a broad molecular weight distribution is provided. More particularly, a process to produce ethylene polymers that have low formation of smoke and odor during blow molding is provided.

FIELD OF THE INVENTION

This invention is related to the field of processes that producepolymers, where said polymers comprise polymerized ethylene. The phrase“ethylene polymers” as used in this application includes homopolymers ofethylene, and copolymers of ethylene with another monomer. Particularly,this invention is related to the field of processes that produceethylene polymers having a broad molecular weight distribution. Moreparticularly, this invention is related to the field of processes thatproduce ethylene polymers that have low formation of smoke and odorduring blow molding.

BACKGROUND OF THE INVENTION

There are many production processes that produce ethylene polymers.Ethylene polymers are utilized in many products, such as, for example,films, coatings, fibers, bottles and pipe. Producers of such ethylenepolymers are continuously conducting research to find improved ethylenepolymers.

Ethylene polymers with a broad molecular weight distribution generallyhave excellent processing characteristics such as, for example, highshear ratio, high shear at onset of melt fracture, low weight and dieswell, and excellent physical properties such as high environmentalstress crack resistance. However, often times, these ethylene polymerscan produce smoke and odors when blow molded into manufactures.

This invention provides ethylene polymers having a broad molecularweight distribution and also low formation of smoke and odors duringblow molding. Due to these improved properties, these ethylene polymersare ideal for blow molding bottles and other manufactures.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a process to polymerizeethylene, or to copolymerize ethylene with at least one other monomer,to produce ethylene polymers.

It is another object of this invention to provide said ethylenepolymers.

It is another object of this invention to provide ethylene polymershaving high environmental stress crack resistance and low formation ofsmoke and odor during blow molding.

It is yet another object of this invention to provide a process to usesaid ethylene polymers to produce a manufacture.

It is still yet another object of this invention to provide amanufacture comprising said ethylene polymers.

In accordance with this invention, a process is provided, said processcomprising polymerizing ethylene, or copolymerizing ethylene with atleast one other monomer, wherein said polymerizing is conducted:

in a loop reactor with isobutane as a diluent;

at a temperature in a range of about 200° F. to about 220° F.;

with a catalyst system comprising chromium and a support;

in the presence of at least one trialkylboron;

wherein the chromium is present in a range of about 1% by

weight to about 4% by weight based on the weight of the support;

wherein said support comprises silica and titania;

wherein said support has a surface area of about 400 m²/gram to about800 m²/gram and a pore volume of about 1.8 ml/gram to about 4 ml/gram;

wherein the titania is present in a range of about 0.5% by weight toabout 3% by weight titanium based on the weight of the support;

wherein said catalyst system is activated at a temperature from about1000° F. to about 1300° F.;

wherein said trialkylboron is represented by the formula, BR₃,

where R is an alkyl group of up to 12 carbon atoms.

In another embodiment of this invention, said ethylene polymers areprovided.

In yet another embodiment of this invention, a process for using saidethylene polymers to produce a manufacture is provided.

In still another embodiment of this invention, a manufacture is providedcomprising said ethylene polymers.

These and other objects of this invention will become more evident fromthe following description and claims.

DETAILED DESCRIPTION OF THE INVENTION

A process comprising polymerizing ethylene, or copolymerizing ethylenewith at least one other monomer is provided. Said “at least one othermonomer” can be olefins having from 4 to about 16 carbon atoms permolecule. Suitable monomers, that can be polymerized with ethylene toproduce copolymers with excellent properties, can be selected from thegroup consisting of 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene,1-octene.

The polymerizing is conducted in a loop reactor process at a temperaturein a range of about 200° F. to about 220° F. with isobutane as adiluent. The loop reactor process is well known in the art and isdisclosed, for instance, in Norwood, U.S. Pat. No. 3,248,179, thedisclosure of which is hereby incorporated by reference.

The polymerizing is conducted using a catalyst system comprisingchromium and a support. The chromium can be any suitable chromiumcompound that facilitates the polymerization of olefins. Suitableexamples of chromium compounds include, but are not limited to, chromiumnitrate, chromium acetate, chromium trioxide, and mixtures thereof. Theamount of chromium present is from about 1% by weight to about 4% byweight. Preferably, the amount of chromium present is from about 1.5% byweight to about 3.5% by weight, most preferably, from 2% by weight to 3%by weight, where such weight percents are based on the weight of thesupport.

The chromium can be combined with the support in any manner known in theart. Examples of combining the chromium with the support can be found inU.S. Pat. Nos. 3,976,632; 4,248,735; 4,297,460; and 4,397,766; theentire disclosures of which are hereby incorporated by reference.

The term “support” is not meant to be construed as an inert component ofthe catalyst system. The support used in the catalyst system of thisinvention comprises (or alternatively, “consists essentially of” or“consists of”) silica and titania. These supports are known in the artand are disclosed in U.S. Pat. Nos. 2,825,721; 3,225,023; 3,226,205;3,622,521; 3,625,864; 3,780,011; 3,887,494; 3,900,457; 3,947,433;4,053,436; 4,081,407; 4,151,122; 4,177,162; 4,294,724; 4,296,001;4,392,990; 4,402,864; 4,405,501; 4,434,243; 4,454,557; 4,735,931;4,981,831; and 5,037,911, the entire disclosures of which are herebyincorporated by reference. However, it should also be noted that thesesupports are available commercially from such sources as the W. R. GraceCorporation.

Generally, the amount of titania present is from about 0.5% by weight toabout 3% by weight titanium. Preferably, the amount of titania presentis from about 0.8% by weight to about 2.6% by weight titanium, mostpreferably from 0.8% by weight to 1.5% by weight titanium, where suchweight percents are based on the weight of the support.

The support has a surface area from about 400 m²/gram to about 800m²/gram. Preferably, the support has a surface area from about 450m²/gram to about 700 m²/gram, and most preferably, from 500 m²/gram to600 m²/gram. Furthermore, the support has a pore volume of from about1.8 ml/gram to about 4 ml/gram. Preferably, the support has a porevolume of from about 2 to about 3.5 ml/gram, and most preferably, from2.3 ml/gram to 3 ml/gram.

The catalyst system used in this invention is activated in accordancewith any manner known in the art that will contact an oxygen containingambient with the catalyst system. Suitable examples of this type ofprocedure can be found in U.S. Pat. Nos. 3,887,494; 3,900,457;4,053,436; 4,081,407; 4,296,001; 4,392,990; 4,405,501; and 4,981,831,the entire disclosures of which are hereby incorporated by reference.Generally, activation is conducted at a temperature in a range of about1000° F. to about 1300° F. Preferably, activation is conducted at atemperature in a range of about 1050° F. to about 1250° F., and mostpreferably, from 1100° F. to about 1200° F. Currently, the preferredoxidizing ambient is air. This activation is carried out for a timeperiod of about 1 minute to about 50 hours. This allows for at least aportion of any chromium in a lower valance state to be converted to ahexavalent state.

The polymerizing is also conducted in the presence of at least onetrialkylboron with a formula, BR₃, where R is an alkyl group of up to 12carbon atoms. Preferably, said trialkylboron is triethylboron (TEB). Theamount of the cocatalyst used in a polymerization, stated in parts permillion by weight based on the weight of the diluent in the reactor, isfrom about 1 part per million to about 6 parts per million. Preferably,it is from about 1.5 parts per million to about 4 parts per million, andmost preferably, it is from 2 parts per million to 3 parts per million.

Hydrogen can be present in the loop reactor to control molecular weight.Currently, about 0 to about 3 mole percent hydrogen can be used.

Generally, said ethylene polymers produced by this process have thefollowing properties: a high load melt index (HLMI) of about 10 to about60 grams per ten minutes; a density of about 0.950 to about 0.960 gramsper cubic centimeter; a shear ratio (high load melt index (HLMI)/meltindex (MI)) of about 100 to about 300, a polydispersity (weight averagemolecular weight (Mw)/number average molecular weight (Mn)) of about 15to about 30; a Environmental Stress Crack Resistance (ESCR) (ConditionA) of greater than 300 hours; xylene solubles of less than 1.0%; and alow molecular weight polymer content of less than 2%. Low molecularweight polymer is ethylene polymer with a molecular weight of less than1000. Test methods to determine these properties are describedsubsequently in the Examples.

It is preferred when said ethylene polymers have a HLMI of about 15 toabout 40 grams per ten minutes, and most preferably, 15 to 30 grams perten minutes.

It is also preferred when said ethylene polymers have a density of about0.952 to about 0.958 grams per cubic centimeter, and most preferably,0.953 to 0.957 grams per cubic centimeter.

It is also preferred when said ethylene polymers have a shear ratio(HLMI/MI) of about 120 to about 200, most preferably, from 130 to 180.

It is also preferred when said ethylene polymers have a polydispersity(Mw/Mn) of about 18 to about 25, and most preferably, of 19 to 25.

It is also preferred when said ethylene polymers have a EnvironmentalStress Crack Resistance (Condition A) of greater than about 400 hours,and most preferably, greater than 500 hours.

It is also preferred when said ethylene polymers have a low formation ofsmoke and odor when blow molded into manufactures as indicated by havingless than 0.85% xylene solubles and less than 1.6% low molecular weightpolymer. Most preferably, said ethylene polymers have less than 0.6%xylene solubles and less than 1% low molecular weight polymer.

Said ethylene polymers can be used to produce manufactures. Saidethylene polymers can be formed into a manufacture by any means known inthe art. For example, said ethylene polymers can be formed into amanufacture by blow molding, injection molding, and extrusion molding.Further information on processing said ethylene polymers into amanufacture can be found in MODERN PLASTICS ENCYCLOPEDIA, 1992, pages222-298. One important application for said ethylene polymers is theproduction of bottles and other manufactures by blow molding.

EXAMPLES

These examples are provided to further illustrate the invention. Thescope of the invention should not be limited to these examples.

Test Methods

A Quantachrome Autosorb-6 Nitrogen Pore Size Distribution Instrument wasused to determined the surface area and pore volume of the supports.This instrument was acquired from the Quantachrome Corporation, Syosset,N.Y.

Polymer density was determined in grams per cubic centimeter (g/cc) on acompression molded sample, cooled at about 150° C. per hour, andconditioned for about 40 hours at room temperature in accordance withASTM D1505 and ASTM D1928, procedure C.

Melt index (MI, g/10 minutes) was determined in accordance with ASTMD1238 at 190° C. with a 2,160 gram weight.

High load melt index (ILMI, g/10 minutes) was determined in accordancewith ASTM D1238 at 190° C. with a 21,600 gram weight.

Environmental Stress Crack Resistance (ESCR, hrs) was determinedaccording to ASTM D1693, Conditions A and B.

The Heterogeneity Index (HI) was determined using size exclusionchromatography (SEC) analyses that were performed at 140° C. on a Water,model 150 GPC with a refractive index detector. A solution concentrationof 0.25 weight percent in 1,2,4-trichlorobenzene was found to givereasonable elution times. To determine smoke potential of the ethylenepolymer, the amount of ethylene polymer in the <1000 molecular weightrange was calculated.

Ethylene polymers obtained by this invention are useful for blow moldingapplications. In these examples, blow molding evaluations were conductedby blowing a one gallon (105.1 gm) bottle on a Uniloy 2016 single headblow molding machine using a 2.5 inch diameter die, 20 degree divergingdie, 32% accumulator position, 8.5 second blow time, 0.10 second blowdelay, 0.75 second pre-blow delay and a 45° F. mold temperature. Areciprocating screw speed of 45 revolutions per minute (rpm) was used,providing parison extrusion at shear rates greater than 10,000/secthrough the die.

Percent weight swell measures the amount a molten polymer expandsimmediately as it exits the die. It is a measure of the “memory” of thepolymer chains as they seek to relax and thus reform the polymer shape.Weight swell is an important parameter as it determines how tight a diegap must be adjusted to provide a constant bottle weight. If a polymerhas high weight swell, the die gap required will be tighter to make aproper part weight.

In so doing, it will require higher stress to push the polymer throughthe die than a lower weight swell polymer. Weight swell is defined asthe ratio of the die gap to the final bottle wall thickness.

Another measurement of swell is die swell or diameter swell. This is theratio of a parison diameter to a die diameter. These numbers arereferenced to a standard commercial blow molding polymer, Marlex®5502polyethylene, obtained from Phillips Petroleum Company.

Bottle stress crack resistance was tested using ten 105 gram one gallonbottles made as described above on a Uniloy 2016 machine. The bottleswere filled with a 10% Orvus-K detergent solution, capped, and placed ina 140° F. hot room. Bottle failures were noted each day, and a 50% meanfailure time was calculated for each set.

Onset of melt fracture of each ethylene polymer was evaluated on thesame Uniloy machine by opening the die gap and extruding the ethylenepolymer. Shear rate was increased steadily by increasing the screw rpm.Onset was the rpm at which the parison showed visible signs of meltfracture, such as a shark skin appearance or a distorted surface.

Two methods to measure a polymer's ease of processibility were utilizedin these examples. The first, listed as “Output” in the tables presentedsubsequently in this disclosure, was calculated from the cycle time ofthe machine and the weight of the bottle and flashing. Thus, thismeasure describes the rate of bottle output in pounds of polymer perhour at which the polymer in question was blow molded into bottlesduring normal operation. Therefore, it is a measure of the commercialrate of bottle production. The second measure of polymer processibilityis listed as “1-Minute Output” in the tables presented subsequently inthis disclosure. It describes the speed at which one part of the blowmolding operation was accomplished. For this test, the extruder on theblow molding machine was set at 45 rpm, and it was allowed to extrudepolymer for one full minute at the same die gap used to make the desiredbottles. After 1 minute, the test was stopped, and the polymer wasweight to determine the 1-minute output value. Thus, the 1-minute outputvalue gives an indication of the rate of extrusion of the polymer duringthe blow molding operation.

Xylene solubles (%) was determined in accordance with ASTM D-5492-94.

Subjective ratings were also made by the operator running the blowmolding machine, in which he judged the degree of smoke and odorproduction by the ethylene polymer during processing, the degree of meltfracture, and the processibility of the ethylene polymer. On this scale,the best ethylene polymers were given a 1 and the worst a 5. Ratings inthe 1 to 3 range were considered acceptable.

Invention Examples 1-5 (Table One)

Ethylene polymers were prepared by contacting a catalyst system withmonomers in a continuous, particle form process which employed a liquidfull 15.2 cm diameter pipe loop reactor having a volume of 23 gallons(87 liters), isobutane as the diluent, and occasionally some hydrogen toregulate the molecular weight of the ethylene polymer produced. The loopreactor was operated to have a residence time of 1.25 hours. The reactortemperature was varied over a range of 95° C. to 107° C., depending onthe particular experiment, and the pressure was four Mpa (580 psi). Atsteady state conditions, the isobutane feed rate was about 46 liters perhour, the ethylene feed rate was about 30 lbs/hr, and the 1-hexene feedrate was varied to control the density of the ethylene polymer. Ethylenepolymer was removed from the loop reactor at the rate of 25 lbs per hourand recovered in a flash chamber. A Vulcan dryer was used to dry theethylene polymer under nitrogen at about 60° C. to 80° C.

Ethylene that had been dried over alumina was used as the monomer.Isobutane that had been degassed by fractionation and dried over aluminawas use as the diluent. Triethylboron (TEB) or triethylaluminum (TEA)was also sometimes used as a cocatalyst as indicated in the tablesbelow.

In example 1 and 4-6, a commercially available catalyst system,designated 963 Magnapore, was purchased from the W. R. GraceCorporation. It had a chromium content of about 1.0 weight percent basedon the weight of the support and about 2.5 weight percent titanium basedon the weight of the support. It had a surface area of about 500 to 550square meters per gram and a pore volume of about 2.4 to 2.6 ml/g.

In examples 2 and 3, another catalyst system was obtained from W. R.Grace, designated Magnapore-1, which was essentially identical to 963Magnapore except that it contained 1.0 weight percent titanium based onthe weight of the support.

In some cases, as indicated in the tables below, extra chromium wasadded to the catalyst system. This was accomplished by impregnating thecatalyst system to incipient wetness or somewhat less, with a methanolsolution of chromium (III) nitrate containing 0.05 grams of chromium per100 milliliters.

In Inventive Examples 1-6, the ethylene polymer produced had broadmolecular weight distributions as shown by having a shear ratio of 145to 250, a polydispersity of 19.8 to 24.7, and a ESCR (Condition A) of321 to greater than 1000 hours. In addition, the ethylene polymers havea low percentage of low molecular weight polymer (0.44%-1.97% byweight), and xylene solubles (0.20%-0.96%), thereby having low smoke andodor when blow molded into manufactures.

Comparative Examples 1-15 (Table Two)

Ethylene polymers were prepared in the same loop reactor and under thesame process parameters as described above except the amount of titaniumand cocatalyst, the surface area, and the pore volume were variedoutside the limits of this invention.

Various catalysts and cocatalysts were used in these runs as indicatedin the table and descriptions below.

In comparative examples 1-5 and 12, a commercially available catalystsystem, designated 964 Magnapore, was purchased from the W. R. GraceCorporation. It had a chromium content of about 0.8 weight percent basedon the weight of the support and about 5 weight percent titanium basedon the weight of the support. It had a surface area of about 550 to 600square meters per gram and a pore volume of about 2.1 to 2.3 ml/g.

In comparative example 6, a catalyst system was obtained from the W. R.Grace Corporation, designated HPVSA indicating its relatively high porevolume and surface area compared to standard 969MS grades. It had asurface area of 577 square meters per gram and a pore volume of 2.21ml/g.

In comparative examples 7-9, a commercially available catalyst system,designated 963 Magnapore, was purchased from the W. R. GraceCorporation. It had a chromium content of about 1.0 weight percent basedon the weight of the support and about 2.5 weight percent titanium basedon the weight of the support. It had a surface area of about 500-550square meters per gram and a pore volume of about 2.4-2.6 ml/g.

In comparative example 9, a commercially available catalyst system waspurchased from the W. R. Grace Corporation. This catalyst system wassold under the name of 965 Sylopore. It had surface area of about 400square meters per gram and a pore volume of about 1.0 ml/g.

In comparative example 10, titanium was added to a 969MS catalyst systemobtained from W. R. Grace Corporation by first drying the 969MS catalystin dry nitrogen in a fluidized bed at 400-500° F., then lowering thetemperature to 250° F.-400° F. during which time titanium isopropoxideliquid was added over a period of about one hour. The titaniumisopropoxide evaporated while transported by the nitrogen in a ⅛″stainless steel coil which introduced the vapor into the bottom of thebed. After all the titanium had been added, the nitrogen gas stream wasreplaced by dry air, and the temperature was increased to the desiredactivation temperature in the usual fashion. The final catalystcomposition was analyzed after activation.

In comparative example 11, a catalyst system was obtained from W. R.Grace Corporation designated HPV silica. It had a surface area of about300 square meters per gram, and a pore volume of about 2.5 ml/g.

In comparative example 13, a commercially available catalyst system waspurchased from the W. R. Grace Corporation called 969MS. This catalysthad a surface area of about 300 square meters per gram, and a porevolume of about 1.6 ml/g.

As can be seen in Table Two, the objects of this invention, such as, lowsmoke and odor potential, high ESCR, low swell, and high shear at meltfracture are not achieved when the catalyst system and operatingparameters of this invention are not met. Thus, in comparative runs 1-5and 12, where the titanium content of the catalyst system was too high,the molecular weight distribution became too broad as indicated by theshear ratio (HLMI/MI) and polydispersity (Mw/Mn) giving excessively highsmoke and odor formation as indicated by high low molecular weightcontent and xylene solubles.

In contrasting runs 6, 11, and 13, where the titania was too low, themolecular weight distribution was not broad enough (HLMI/MI and Mw/Mn)which led to poor ESCR, swell or output.

Likewise, surface area, pore volume, activation temperature, andcocatalyst are also varied in the table with the result that desiredethylene polymer properties were not obtained if the catalyst system andoperating parameters were varied outside of the prescribed bounds ofthis invention. In Run 8, cocatalyst was not used, and the ethylenepolymer produced had a low ESCR. In Runs 9 and 10, a catalyst systemhaving lower surface area and pore volume than specified in theinvention produced ethylene polymer having a lower shear ratio and ESCR(Condition A). Chromium content affects swell, as does surface area andpore volume. Cocatalyst affects ESCR, as does activation temperature.Titanium content affects the production of smoke when the ethylenepolymer is blow molded into manufactures.

TABLE ONE Invention Example No. 1 2 3 4 5 6 % Titanium (Ti) 2.5 1.0 1.02.5 2.5 2.5 % Chromium (Cr) 2.0 2.0 2.0 2.0 2.0 2.0 Surface Area 540 560560 540 540 540 (SA) (m²/g) Pore Volume 2.6 2.7 2.7 2.6 2.6 2.6 (PV(ml/g) Activation Temp. 1100 1100 1200 1200 1100 1000 (° F.) CocatalystTEB/ TEB TEB TEB TEB TEB TEA Cocatalyst 2.3/2.3 1.94 2 2.05 2 2 (ppm byweight) Productivity (g/g) 1504 6250 1558 7143 5882 7692 HLMI (g/10min.) 21.8 15.2 19.7 12.5 17.2 19.2 Shear Ratio 145 217 164 250 191 175(HLMI/MI) Density (g/ml) 0.9547 0.9557 0.956 0.9537 0.9559 0.9545Polydispersity 22.7 19.8 22.8 23.4 21.4 24.7 (Mw/Mn) ESCR-A (hrs) 1000441 321 >1000 >1000 >1000 ESCR-B (hrs) 100 120 76 180 144 241 BottleESCR (hrs) NA NA NA NA >700 >700 Die Swell (%) 46 42.9 45 40.3 44.9 46.4Weight Swell (%) 409 338 404 386 414 436 Shear Rate at 1504 1183 15582090 2234 2227 onset of Melt Fracture (sec⁻¹) Output (lbs/hr) 81.6 86.186.1 79.8 71 54.8 % Low Molecular 0.85 0.91 0.44 1.58 1.43 1.97 Weight(<1000) Smoke Rating 2 2 2 3 3 3 (1-5) Melt Fracture 2 2 2 3 2 2 Rating(1-5) Odor Rating 3 3 3 3 3 3 (1-5) 1 minute Output 915 711 1125 12571343 1339 (grams) Xylene Solubles 0.20 0.56 0.52 0.92 0.80 0.96 (%)

TABLE TWO Comparative Example #: 1 2 3 4 5 6 7 Deviative Variable HighTi High Ti High Ti High Ti High Ti Low Ti TEA % Titanium (Ti) 5 2.5 5 55 0 2.5 % Chromium (Cr) 3 2 1 2 3 2 2 Surface Area (SA) (m²/g) 555 555555 550 555 577 540 Pore Volume (PV) (ml/g) 2.11 2.11 2.11 2.26 2.112.21 2.6 Activation Temp. (° F.) 1000 1000 1000 1100 1000 1000 1100Cocatalyst TEB TEB TEB TEB TEB TEB TEA Cocatalyst (ppm by weight) 2 2 22 4 2 2 Productivity (g/g) 5882 7813 3571 5556 5882 8333 4167 HLMI (g/10min.) 17.8 21 17.2 14.9 17.4 19.3 17 Shear Ratio (HLMI/MI) 222 191 191248 291 138 113 Density (g/ml) 0.9552 0.9554 0.9567 0.9545 0.9564 0.95410.9533 Polydispersity (Mw/Mn) 33.5 38.1 38.8 41.9 43.9 14.4 24 ESCR-A(hrs) >1000 >1000 >1000 >1000 >1000 395 400 ESCR-B (hrs) 300 317 429 170261 73 85 Bottle ESCR (hrs) >700 >700 >700 >700 >700 >700 >700 Die Swell(%) 38 40.1 42.6 43.9 40.1 45.4 45.1 Weight Swell (%) 410 392 395 434457 375 325 Shear Rate at onset of Melt Fracture 2194 2155 2200 21962169 2241 1031 (sec⁻¹) Output (lbs/hr) 59.3 70.2 56.1 58.8 58 85.2 82 %Low Molecular Weight (<1000) 2.42 2.34 2.44 3.73 2.92 0.62 1.22 SmokeRating (1-5) 4 4 3 3 4 2 2 Melt Fracture Rating (1-5) 4 4 2 2 4 2 2 OdorRating (1-5) 5 5 4 3 5 2 2 1 minute Output (grams) NA NA NA 1321 NA 1348620 Xylene Solubles (%) 1.20 0.80 1.07 1.12 1.10 NA 0.6 ComparativeExample #: 8 9 10 11 12 13 Deviative Variable no TEB Low PV Low SA LowTi and SA Hi Ti and TEA All Variables % Titanium (Ti) 2.5 2.5 3 0 5 0 %Chromium (Cr) 2 1 1 2 2 1 Surface Area (SA) (m²/g) 540 400 300 300 555285 Pore Volume (PV) (ml/g) 2.6 1 1.6 2.5 2.11 1.5 Activation Temp. (°F.) 1100 1100 1250 1200 1100 1450 Cocatalyst (ppm by weight) None TEBTEB TEB TEA None Conc. (ppm) 0 2.1 6.2 2 2 0 Productivity (g/g) 166672600 9091 10000 2000 HLMI (g/10 min.) 20.1 22.9 37.3 15.7 16.7 26.2Shear Ratio (HLMI/MI) 87 81 128.6 157 139 79 Density (g/ml) 0.9586 0.9550.958 0.9544 0.9545 0.9547 Polydispersity (Mw/Mn) 12.6 16.8 22 14.2 30.26.3 ESCR-A (hrs) 209 170 200 248 304 <115 ESCR-B (hrs) 45 92 50 61 102<50 Bottle ESCR (hrs) NA 245 180 508 >700 114 Die Swell (%) 51 44 41.344.1 39.3 Weight Swell (%) 353 35 334 330 375 Shear Rate at onset ofMelt Fracture 1014 601 2437 1624 2460 (sec⁻¹) Output (lbs/hr) 85.9 84.583.7 81.4 % Low Molecular Weight (<1000) 0 0.51 2.83 0 Smoke Rating(1-5) 2 2 3 2 Melt Fracture Rating (1-5) 2 2 3 2 Processing (1-5) 3 2 32 Odor Rating (1-5) 3 2 3 1479 1 minute Output (grams) 428 1465 976Xylene Solubles (%) 0.28 NA 0.75

That which is claimed is:
 1. A process comprising polymerizing ethylene,or copolymerizing ethylene with at least one other monomer, wherein saidpolymerizing is conducted: in a loop reactor with isobutane as adiluent; at a temperature in a range of about 200° F. to about 220° F.;with a catalyst system comprising chromium and a support; in thepresence of a trialkylboron; wherein the chromium is present in a rangeof about 1% by weight to about 4% by weight based on the weight of thesupport; wherein said support comprises silica and titania; wherein saidsupport has a surface area of about 400 m²/gram to about 800 m²/gram anda pore volume of about 1.8 ml/gram to about 4 ml/gram; wherein thetitania is present in an amount sufficient to give titanium in a rangeof about 0.5% by weight to about 3% by weight based on the weight of thesupport; wherein said catalyst system is activated at a temperature fromabout 1000° F. to about 1300° F.; wherein said trialkylboron isrepresented by the formula BR₃, where R is an alkyl group of up to 12carbon atoms; and wherein the process produces an ethylene polymerhaving the following properties: a high load melt index of 15 to 40grams per ten minutes; a density of 0.952 to 0.958 grams per cubiccentimeter, a shear ratio (high load melt index/melt index) of 120 to200, a (weight average molecular weight/number average molecular weight)of 18 to 25, an Environmental Stress Crack Resistance (ASTM D1693,Condition A) of greater than 300 hours, less than 0.85% xylene solublesand less than 1.6% low molecular weight polymer.
 2. A process accordingto claim 1 wherein the amount of chromium present is from about 1.5% byweight to about 3.5% by weight, where such weight percents are based onthe weight of the support.
 3. A process according to claim 2 wherein theamount of titania present is from about 0.8% by weight to about 2.6% byweight titanium, where such weight percents are based on the weight ofthe support.
 4. A process according to claim 3 wherein the support has asurface area from about 450 m²/gram to about 700 m²/gram.
 5. A processaccording to claim 4 wherein the support has a pore volume from about 2to about 3.5 ml/gram.
 6. A process according to claim 5 wherein saidactivation is conducted at a temperature in a range of about 1050° F. toabout 1250° F.
 7. A process according to claim 6 wherein the amount ofthe cocatalyst used in a polymerization, stated in parts per million byweight based on the weight of the diluent in the reactor, is from about1.5 parts per million to about 4 parts per million.
 8. A processaccording to claim 7 wherein the amount of chromium present is from 2%by weight to 3% by weight, where such weight percents are based on theweight of the support.
 9. A process according to claim 8 wherein theamount of titania present is from about 0.8% by weight to 1.5% by weighttitanium, where such weight percents are based on the weight of thesupport.
 10. A process according to claim 9 wherein the support has asurface area from 500 m²/gram to 600 m²/gram.
 11. A process according toclaim 10 wherein the support has a pore volume of from 2.3 ml/gram to 3ml/gram.
 12. A process according to claim 11 wherein said activation isconducted at a temperature in a range of 1100° F. to 1200° F.
 13. Aprocess according to claim 12 wherein the amount of the cocatalyst usedin a polymerization, stated in parts per million by weight based on theweight of the diluent in the reactor, is from 2 parts per million to 3parts per million.
 14. A process according to claim 13, wherein saidtrialkylboron is triethylboron.
 15. A process according to claim 14wherein said other monomer is selected from the group consisting of1-butene, 1-pentene, 4-methyl-1-pentene, 1 -hexene and 1 -octene.
 16. Aprocess according to claim 15 wherein said polymerizing is conducted inthe presence of hydrogen.
 17. A process according to claim 15 whereinsaid other monomer is 1-hexene.
 18. A process according to claim 1wherein said catalyst system consists essentially of said chromium andsaid support.